You can be forgiven for not having heard of Markdown. Not everyone has. But those who have, and that includes myself, happen to think it's a really neat idea that brings the concept of text formatting to plain text files.
To understand why there's a need for Markdown, you first need to understand about text file formats and their impenetrability. Word processor formats create document files which are totally incomprehensible unless you happen to own the correct word processor, although the power that you can exercise over the layout of your text is immense.
On the other end of the scale, plain text files are small, portable, and easy to understand, but there's no support for any formatting such as headings or italics.
HTML brings a degree of formatting (known technically as markup) to plain text files but, while a web browser can easily understand HTML, mere humans often can't. So what's needed is something which works a bit like HTML but which is much simpler. And that is precisely where Markdown (the opposite of markup, presumably) fits in.
In Markdown, you specify a big heading by putting a hash sign in front of the text. For a slightly smaller heading, use 2 hashes. For a horizontal line, use 3 dashes. What you end up with is a file that can be edited in any word processor or text editor on the planet, but which works best if you use a Markdown-aware editor.
One of the best such editors for Windows is Markdown Pad. You can get it from http://markdownpad.com/ and the basic version is free of charge. As you can see from the screen shot below, it's split into 2 windows, so that you can edit the text and the markdown on one side of the screen and instantly see your formatted results on the other.
One of the benefits of Markdown is that it works across many platforms. For example, if you want to edit a Markdown document on your iPhone or iPad, you can use a suitable Markdown-aware app and you know that the formatting will all be neatly preserved. Or if there isn't a suitable app for your device, just use any text editor and all you'll lose is the formatted preview.
Markdownpad is a 30 MB download and, according to Web of Trust and VirusTotal, is malware-free, so you may like to try it. It should run on Windows XP and above.